1. Field of the Invention
The present invention relates to a cleaning blade, and an image forming apparatus and a process cartridge using the cleaning blade.
2. Description of the Related Art
In image forming apparatuses using electrophotography, images are formed through processes of charging, irradiation, development, transfer etc. applied to an image bearing member.
Ordinarily, corona products produced in the charging process that remain on the surface of the image bearing member, and toner or its components remaining on the surface of the image bearing member after the transfer process, are removed by a cleaning process.
In terms of improving the reliability of the image forming apparatus, an image bearing member formed of an inorganic material such as amorphous silicon, or a highly durable image bearing member that has a surface layer in which an acrylate-based material or inorganic particulates are dispersed, are now widely used. These image bearing members, such as photoreceptors, have excellent abrasion resistance against a cleaning blade. Therefore, such image bearing members have by far a longer working life than once typically used image bearing members, resulting in significantly less frequent maintenance and replacement, which is greatly preferable in terms of impact on the environment and cost savings.
However, although successful in prolonging the working life of the image bearing member, the frequency of replacement thereof remains unchanged unless the working life of the cleaning blade is also prolonged. In addition, in the type of image forming apparatus of late, use of a process cartridge in which members such as the image bearing member and the cleaning blade are combined into a single integrated unit has become popular because it facilitates maintenance of the image forming apparatus. Therefore, the working life of the cleaning blade becomes a bottleneck, in that it necessitates replacement of the entire of the process cartridge even though the image bearing member is not completely worn out.
The cleaning blade is typically formed of an elastic body such as polyurethane rubber with a reed-like shape and removes toner remaining on the image bearing member by pressing the ridge line of the front end of the cleaning blade against the circumference surface of the image bearing member while supporting the base end of the cleaning blade with a supporting member.
In addition, an image forming apparatus using a toner having a form significantly close to a sphere with a small particle diameter manufactured by a polymerization method, etc. has been introduced into the market to meet recent demand for improvement of image quality. This polymerized toner has excellent transfer efficiency in comparison with typical toner and is capable of satisfying demand for improved image quality. However, the polymerized toner is difficult to sufficiently remove from the surface of the image bearing member with the cleaning blade, thereby causing a problem with regard to the cleaning performance. This difficulty of removal occurs because the polymerized toner particles have a near-spherical form and are of such a small size that they slip through the small gap between the cleaning blade and the image bearing member.
One way to prevent such slip-through of the toner is to increase the contact pressure between the image bearing member and the cleaning blade. However, as illustrated in FIG. 8A, if the contact pressure of the cleaning blade is increased, the friction between an image bearing member 23 and a cleaning blade 262 increases so that the cleaning blade 262 is drawn in the moving direction of the image bearing member 23, resulting in turning inward or outward of a front edge portion 262c of the cleaning blade 262.
If cleaning continues with the front edge portion 262c turned inward or outward, the cleaning blade is easily abraded, which creates a locally abraded portion several μm from the front edge portion 262c of a front end surface 262a of the cleaning blade 262 as illustrated in FIG. 8B. If cleaning still continues in this state, the locally abraded portion increases, and finally leads to chipping of the front edge portion 262c as illustrated in FIG. 8C. Once part of the front edge portion 262c has chipped off, the cleaning blade no longer removes the toner sufficiently, resulting in poor cleaning performance, meaning the end of the working life of the cleaning blade. A bottom surface 262b of the cleaning 262 is perpendicular to the front end surface 262a. 
To prolong the working life of the cleaning blade, for example, Japanese patent application publication no. 2005-107376 (hereinafter referred to as JP-2005-107376-A) describes a polyurethane elastomer cleaning blade having a layer formed of isocyanate with a thickness of 0.1 mm at the portion of contact with the image bearing member. However, although suitable for an image bearing member having a smooth surface made of a material such as amorphous silicon, this cleaning blade chips off at an early stage and causes poor cleaning performance in most cases when it cleans a roughened surface of an image bearing member such as an organic photoconductor using inorganic particulates for the surface layer to improve the cleaning property and/or the abrasion resistance.
JP-2000-66555-A describes an image forming apparatus that has a decreased friction force between the image bearing member and the cleaning blade, accomplished by providing a low-friction layer to the cleaning blade. This cleaning blade demonstrates extremely good cleaning performance at an initial stage before a great number of images are formed.
However, as image formation processes are repeated, the low-friction material on the uppermost surface of the low-friction layer drops off, resulting in an increase in the abrasion force between the image bearing member and the cleaning blade, which leads to localized chipping of the cleaning blade. Consequently, poor cleaning is unavoidable.
Japanese patent no. 3602898 (hereinafter referred to as JP-3602898-B) describes a polyurethane elastomer cleaning blade. At least the contact portion of the cleaning blade with an image bearing member is manufactured by impregnation of a urethane acrylate monomer followed by irradiation of UV light for curing. This cleaning blade is suitably used at an initial stage before a number of images are formed. However, as image formation is repeated, in many cases the top of the cleaning blade contacting the image bearing member chips off, resulting in poor cleaning performance.
The hardness of the urethane acrylate monomer described in JP 3602898 cannot be measured once it infiltrates the polyurethane elastomer. Therefore, the urethane acrylate monomer is applied to a glass plate and irradiated with UV light to form a cured layer. The cured layer is confirmed to have a sufficient hardness. However, the hardness of the polyurethane elastomer which is impregnated with the urethane acrylate monomer followed by irradiation of UV light is unexpectedly insufficient and is almost the same as or softer than the hardness of untreated polyurethane elastomer.
The present inventors have carefully investigated the poor performance of the cleaning blade described in JP 3602898 and found that most of the urethane acrylate monomer that has infiltrated the polyurethane elastomer is not polymerized but forms an oligomer having a small molecular weight.
The urethane acrylate monomer is polymerized in the processes of producing radicals by absorption of UV light by a polymerization initiator and opening up the C—C double bond of the urethane acrylate monomer in a chain reaction.
The life of a radical is typically only several tens of nano-seconds, and most of the radicals are deactivated by oxygen, etc. If a monomer is present around the radical produced by irradiation of sufficient amount of UV light, polymerization smoothly proceeds. However, in the case of the urethane acrylate monomer that has infiltrated the cleaning blade, since the UV light penetrates the urethane acrylate monomer via the polyurethane elastomer, the amount of radicals produced is extremely small.
In addition, the concentration of the urethane acrylate monomer is thin. Therefore, most of the urethane acrylate monomers are not polymerized and it is found that the polymerized area reaches a depth of only 3 to 4 μm at most.
Since non-reacted urethane acrylate monomers and oligomers in the polyurethane elastomer weaken the polyurethane elastomer, the cleaning blade easily chips off at even a slight abrasion, which leads to poor cleaning performance.